it will be the same as a tonneu cover.
I used this method about 8 yrs ago on a Mazda B2200 longbed (hard to find hard tonneu covers in that size... very very hard.... think impossible)
I had to haul several things that wouldnt fit under the tonneu, but I couldnt leave out in open air (SWFL gets rain at the most inopportune moments....)
since some of the things weighed in around 100-150lbs, it made since they needed to be at the cab side, hence the initial design.
anyways, I used a piece of plywood, that I attached canvas on either side, that was push buttoned to the bed.
it came out to be almost identical to what you have posted.
I saw no difference in mileage at all over and above the soft tonneu I was using previous.